The present invention relates to magnetoresistive (MR) sensing mechanisms, which may for example be employed in information storage systems or measurement and testing systems.
FIG. 1 shows a cutaway top view of a step in the fabrication of a prior art magnetoresistive (MR) sensor that may for example be used in a head of a disk drive. One or more MR layers 20 that vary in resistance in response to an applied magnetic field have been formed on a wafer, and then coated with a mask that has two openings separated by a small space where a MR sensor layers 20 will remain. After ion beam etching (IBE) that removes the MR layers 20 in the openings, metal bias and lead layers are deposited on the mask and openings, the mask and the metal layers atop it are removed by chemical etching, and the leads 22 remain covering the bias layers. The metal bias layer may be made of a hard magnet such as cobalt that has relatively high resistivity. The lead layer 22 may be made of a physically hard but somewhat resistive material such as tantalum or chromium, or may be a highly conductive material such as gold, which is capped with a tantalum or chromium adhesion layer. The MR layers 20 that remain between the leads 22 will define the track width of the sensor, which may be less than one micron.
FIG. 2 shows a step subsequent to that shown in FIG. 1, the subsequent step defining a height of the sensor, sometimes called the stripe height. A second mask has been created that substantially covers the lead layers 22 as well as covering part of the MR layers 20 disposed between the lead layers. An IBE is then performed that removes the MR layers 20 in areas not covered by the mask, leaving the MR layers 20 only in small region between the leads 22, and removing part of the leads 22 that are not covered by the mask, as shown by dashed outline 25 of the original leads.
For the situation in which the leads are made of a physically soft material such as gold, the dashed outline 25 may represent an edge of a hard bias layer that is exposed after the gold has been milled away by IBE. For the case in which the leads are formed of physically hard materials such as tantalum or chromium, outline 25 may represent remnants of the lead layer. In either case the region outside the unmilled leads 22 and within dashed line 25 consists of material having relatively high resistivity. Later, after additional layers have been formed, the wafer will be cut and polished to line 27, leaving MR sensor 30 connected between the leads 22 that have not been removed by IBE.
FIG. 3 shows an opened up view of the sensor 30 after lapping or polishing of surface 27. Leads 22 can be seen to have long strips 33 that are connected to the sensor 30. The scale of FIG. 3 is magnified compared to that of FIG. 2, displaying the rounding of the leads 22 where the strips connect to larger lead section, the rounding due to limitations in photolithography. Similar limitations prevent the length of the strips, which may each be about one micron, from being shortened without introducing error or imperfections in defining the height of the sensor.
Even for the case in which the leads 22 are made of gold, the small cross sectional area and long length of the strips 33 causes measurable resistance. Since the MR sensor 30 measures a change in resistance, the lead resistance lowers the signal-to-noise ratio of the sensor. For the situation in which the leads are made of tantalum or chromium, this parasitic resistance may be worse.
FIG. 4 shows a view of surface 27 of the completed prior art sensing device of FIG. 3. The MR structure is shown generally at 30 and the leads are shown generally at 22, each being composed of plural layers in this example. A first magnetically soft shield layer 50 has been formed of permalloy, followed by a dielectric read gap layer 52 made of alumina. An antiferromagnetic (AF) layer 55 has been formed on the read gap layer 52, followed by a permalloy pinned layer 58, so that the AF) layer 55 fixes the magnetic direction of the pinned layer 58. A spacer layer 60 has been formed of copper on the pinned layer 58, and a permalloy sense layer 62 has been formed on the spacer layer 60.
A mask was formed atop the sense layer 62, as described with reference to FIG. 1 above, and MR structure 30 defined by milling that extends slightly into the first read gap layer 52. With the mask still present a seed layer 64 of chromium was formed to a thickness of 50 Å, followed by a 600 Å cobalt-based layer 66 that provides magnetic bias to edges of the free, layer 55. A 100 Å tantalum adhesion layer 68 is disposed on the bias layer 66, and a 600 Å gold lead layer 70 formed on the adhesion layer 68. A 100 Å tantalum capping layer 72 was deposited on the gold layer 70, after which the mask was chemically removed, lifting off the metal layers that were formed atop the mask, and a second dielectric read gap layer 75 made of alumina is deposited. For the situation in which metal layers such as bias and lead layers are thickly deposited, the metal layers may completely envelope the mask in an area over the MR sensor 30. The metal coated mask can then be broken off, for example by ultrasonic agitation of an etchant, but this can leave metal fences protruding above the edges of the sensor 30.
A second magnetically soft shield layer 77 has been formed of permalloy atop the read gap layer 75. The shield layers 50 and 77 help to shield the MR structure 30 from magnetic flux originating from parts of a magnetized media track that are not substantially aligned with MR structure 30, allowing the MR structure 30 to more clearly sense the flux from bits that are aligned with MR structure 30. The spacing between shield layers 50 and 77 determines the focus of the MR structure 30 on the magnetic flux emanating from the media directly opposite the MR structure 30, by eliminating magnetic flux that emanates from bits that are not aligned with MR structure 30. For reading high density magnetic patterns it is therefore advantageous to reduce the thickness of the various layers between the shield layers 50 and 77, limiting the thickness of the leads 22. If read gap layer 52 is made too thin, however, lead or stripe etching may create a short circuit to shield layer 50. If read gap layer 75 is made too thin, metal fences protruding above edges of sensor 30 may create a short circuit to shield layer 77. Stated differently, the leads 22 may have a total thickness of about 800 Å, and the MR sensor 30 layers may have a combined thickness of about 500 Å, so that the lead thickness and bias layer thickness can be limiting factors in shield-to-shield spacing.